Comment on the Use of Ivlev's Electivity Index with Planktivorous Fish

Abstract

Application to natural fish populations of Ivlev's equation of electivity, by which the degree of choice a predator exhibits in its feeding behavior is determined by comparing the proportion of a particular prey species present in the environment with the proportion present in the gut, has frequently depended upon the incorrect assumption that both predaceous fish and their zooplankton prey are evenly or randomly distributed throughout a water body. In fact, zooplankton tend to clump both vertically and horizontally, and the use of a vertically towed net to determine zooplankton density does not accurately indicate the zooplankton species composition available to a fish feeding at a particular location. Certain zooplankton sampling equipment also may show selectivity or may not capture particular species, so that a difference between the percent species composition of the plankton sample and that of the fish gut may be due to sampling bias rather than fish electivity. Consideration of these problems will enable field researchers to gather more accurate data concerning fish feeding electivity.